The invention relates to a fuel cell which includes, in addition to a cathode-electrolyte-anode unit, a structure which distributes fuel gas over the electrode surface, with the surface of the structure facing the electrode being formed by a perforated foil.
German Patent Document DE 43 40 153 C1, for example, discloses a solid oxide fuel cell (SOFC) of the generic type, in which, in addition to an independently prefabricated cathode-electrolyte-anode unit, an independent intermediate layer is provided between the electrodes and the bipolar plates or separating plates (known to a person skilled in the art). This independent intermediate layer is constructed as an electrically conductive, elastic and gas-permeable “contact cushion” with a deformable surface structure. The so-called cushion filling may be a highly elastic metallic tissue, and the cushion cover may, for example, be a perforated metal sheet. The purpose of the contact cushion is to provide an optimal electrical contact of the electrodes, while a less exact surface quality of, for example, the anodes and cathodes is acceptable, without impairing electrical contacting within the fuel cell or fuel cell stack constructed of several individual fuel cells.
An essential criterion for an optimal fuel cell function, however, is not merely a sufficient electrical contact of the individual cell elements, but in addition a good flow of fuel gas (or “working gas”) onto the fuel cell electrodes. A good flow of oxygen or ambient air is almost equally important.
One object of the present invention to provide an improved fuel cell which, in addition to a cathode-electrolyte-anode unit, has a structure for distributing the fuel gas over the electrode surface, with the surface of the structure facing the electrode being formed by a perforated foil.
This and other objects and advantages are achieved by the fuel cell structure according to the invention, in which the longitudinal axes of at least some holes or passage openings forming the perforation of the perforated foil are inclined with respect to the foil surface.
With regard to operation of the fuel cell, it is extremely advantageous for the oncoming flow direction of the fuel gas or of the air-oxygen toward the respective electrode to extend at least partially diagonally with respect to the electrode surface, in order to ensure a good substance supply to the electrode. Therefore, according to the invention, the longitudinal axes of holes or passage openings which form the perforation in the above-mentioned (known) foil (on whose surface an electrode of the fuel cell rests) are inclined with respect to the foil surface.
According to a feature of the invention, the longitudinal axes of at least some of the holes or passage openings forming the perforation may be inclined differently with respect to the foil surface, depending on the point at which the holes or passage openings are situated with respect to the fuel cell or the fuel gas flow carried therein. Thus, on the side nearest the fuel gas inlet or feed into the fuel cell, the outlet openings of the hole facing the electrodes may, as a matter of priority, be oriented at least slightly toward the side of the fuel gas outflow from the fuel cell, so that a targeted fuel gas flow occurs also along the electrode surface and the fuel gas is therefore relatively uniformly distributed over the electrode surface. In the same sense, on the side nearest the fuel gas outflow from the fuel cell, the inlet openings of the hole or passage openings which face the electrode (virtually leading away the burnt gas or the reaction products from the electrode) as a matter of priority, may then be oriented at least slightly toward the side of the fuel gas inflow (=fuel gas feed) into the fuel cell. In this fashion, the gas can optimally enter into these passage openings. The term “as a matter of priority” used above means that not all holes in the mentioned areas need be inclined in the above-described manner; rather, only some of the holes may be inclined in this fashion and other holes in this area may have a different inclination or no inclination at all with respect to the foil surface.
However, it is also advantageous to provide holes or passage openings whose longitudinal axes are inclined (and which could also be called bores), also on the side of the fuel gas inlet (or fuel gas feed) into the fuel cell if the inlet openings of these inlet-side holes (again as a matter of priority) are at least slightly oriented toward the fuel gas inlet. In the same manner, on the side of the fuel gas outflow from the fuel cell, the outlet openings of these outlet-side holes (also as a matter or priority) may be oriented at least slightly toward the fuel gas outflow (or toward the side of the fuel gas removal). This technique also helps to provide an optimized fuel gas guidance, because the fresh fuel gas can easily enter into the passage openings, and the burnt gas or the corresponding reaction products can easily exit the passage openings for the fuel gas outflow (fuel gas removal) from the fuel cell. If, in this case, the respective passage openings have longitudinal axes extending in a straight line, advantageously, the respective passage openings can meet both the conditions described in this paragraph and the conditions described in the preceding paragraph, at the same time.
However, it is also possible that holes, whose inlet opening facing away from the electrode is at least slight oriented toward the side of the fuel gas feed, and holes, whose outlet opening facing away from the electrode is at least slightly oriented toward the side of the fuel gas outlet of the fuel cell, are provided essentially alternately (side-by-side) in the foil. As a result, virtually side-by-side, reaction products can be removed from the electrode, and new fresh fuel gas can be guided to the electrode.
According to the invention, holes or passage openings whose longitudinal axes are inclined over the foil surface in a foil of a fuel cell on which an electrode of the fuel cell rests, have another advantage, specifically in regard to a special fuel cell manufacturing process. While, in the fuel cell according to the above-mentioned German Patent Document DE 43 40 153 C1, the ceramic layers of the solid-oxide fuel cells are produced individually (for example, by means of sintering green products of the respective layers—specifically the cathode, the electrolyte and the anode, it is also possible to spray the individual electrode layers successively onto metallic or ceramic substrates. Suitable thermal spraying methods can be used for this purpose, such as vacuum plasma spraying, atmospheric plasma spraying, flame spraying and others. In this case, the so-called substrate can form the carrying structure of the fuel cell, particularly in the case of vacuum-plasma-sprayed fuel cells. Porous and thus gas-permeable as well as simultaneously electrically conductive (and therefore usually metallic) materials can be used for the substrate, in order to ensure as best as possible ensure the supply of the starting material, the removal of the product and the electric current conduction within the fuel cell.
When the perforated foil is used as such a carrier substrate for the electrode layers (to which the first electrode layer is therefore applied in a pulverized manner by a thermal spraying method), if the holes are inclined with respect to the foil surface (in the case where the spraying coating process takes place at a right angle with respect to the foil or substrate surface), good powder retention can be achieved without making the diameter of the holes significantly smaller than the diameter of the drains of the electrode powder to be applied. That is, undesirable penetration of the powder grains during their thermal spraying onto the foil through the holes provided therein can be largely avoided because the longitudinal axes of the holes are inclined with respect to the foil surface.
The perforation in the foil (the holes, passage openings or bores) can be produced, for example, by means of a laser beam, an electron beam or nuclear track. However, these holes can also be formed by electrochemical methods or by masking and etching. All of the above-mentioned methods are well suited for a large-batch production. Particularly for electrode coating onto the foil by a thermal spraying method, the holes are preferably made from the side of the foil or of the substrate that is not to be coated with the electrode material. The reason is that the elevations of the hole edges forming on the coating side for the electrode material will then be helpful for a better interlocking of the ceramic electrode with the foil (or a corresponding strip).
According to an advantageous further development of the invention, the foil perforated according to the invention or a corresponding metal sheet or strip may consist of a suitable metallic material and may be combined with another structure to form an enclosure having a cavity. This additional structure also consists of a metallic material, so that this combined formation can finally form a bipolar plate of the individual fuel cell onto which (or more precisely onto its perforated foil) an electrode layer of the cathode-electrolyte-anode unit can be applied virtually directly, for example, by means of a thermal spraying method. By way of the above-mentioned cavity of this enclosure, the fuel gas or the air-oxygen can then be guided to and from the electrode. This enclosure is therefore a hollow body which preferably consists of a top shell and of a bottom shell that are preferably mutually welded together; they are generally connected in a material-locking manner, along their edges, in order to ensure a sufficient gastightness in this area.
Essentially two embodiments are possible in this case. According to a first variant, the preferably previously perforated foil can be welded as a substrate into a so-called top shell of the enclosure, which may have a rectangular, square, round or arbitrarily oval cutout. For this purpose, a corresponding foil, a strip or a metal sheet is first perforated (in a strip shape or piece by piece), and is then welded into a corresponding cutout of the top shell of a fuel cell enclosure to be produced. Such a weld seam replaces the sealing-in of the fuel cell in its bipolar plate by means of a glass solder or other ceramic or metallic glue, which is normally required in a planar solid-oxide fuel cell.
Subsequently, in the manufacturing method according to the invention, the cathode-electrolyte-anode unit can be sprayed directly onto the welded-in perforated metal foil. In this case, using a spraying mask, the anode is preferably sprayed on, almost to the weld seam between the perforated foil and the top shell. Subsequently, a larger mask is used to apply the electrolyte layer to the anode layer, so that the latter can be made gastight and can be electrically insulated. In addition, the weld seam and a small edge of the metal sheet situated around it can be sealed off by spraying by means of electrolyte material in the same operation. Then, by means of a mask, the cathode layer can be applied by spraying exactly to the surface of the anode.
According to a second embodiment, the top shell can be used directly as a carrier substrate for the anode layer to be sprayed on, in which case it has no recess. Instead, the top shell itself is perforated in the area of the anode (which is to be applied later by means of a thermal powder spraying process), so that the enclosure top shell itself is the foil perforated according to the invention. The perforation can be performed either before or after the two enclosure halves (the top and bottom halves) are welded together. The cathode-electrolyte-anode unit is then applied in the manner as described for the first embodiment. However, the sealing function of the electrolyte will be limited to the sealing-off of the porous anode layer.
In a known manner, several such enclosures can be arranged above one another to form a fuel cell stack. Each fuel cell is provided on its so-called top shell with a cathode-electrolyte-anode unit, the assembled top shell and bottom shell each operating is a bipolar plate, within which the fuel gas can be guided to the anode layer. Since it is then necessary to create a gas distribution space or flow space for the ambient air (or the air/oxygen) between the exterior of the bottom shell of a first enclosure and the top electrode layer of the second enclosure situated in the fuel cell stack below this first cassette, the exterior side of the bottom shell can be provided with a corresponding embossing structure creating such a flow space. The corresponding embossings may, for example, have a meandering structure, interrupted and laterally offset channels, an inflow zone, and much more in the flow direction and transversely to the flow direction.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.